My invention relates to apparatus for data transfer with magnetic storage media such as those commonly known as floppy disks, and more specifically to such apparatus capable of writing on a magnetic disk at two different track densities (i.e., different numbers of tracks per unit distance in a radial direction of the disk). Typically, one of the two track densities is 96 tracks per inch (tpi), and the other is a double density of 192 tpi, although I envisage within my invention any such two track densities that one is an integral submultiple of the other.
A variety of track densities have now been standardized, among them being 96 tpi and 192 tpi, and magnetic disk drives for such various track densities are available on the market. Usually, 96 tpi disk drives employ magnetic head assemblies of the tunnel erase or equivalent type for trimming the opposite edges of each record track as data is written thereon. This type of head asemblies are not used with 192 tpi disk drives; instead, for the required higher track density, this type of disk drive incorporates a close loop servo system using embedded servo patterns on the magnetic disk. The servo patterns or bursts enable the electronics of the 192 tpi disk drive to constantly update the position of the head assemblies, compensating for any thermal hygroscopic expansion or contraction of the disk. The servo system is generally believed to reduce the possibility of head mistracking to a minimum, making it possible to accurately read data from tracks which have not been trimmed and which are held close to each other. Reference may be had to "Kodak 3.3 Flexible 51/4" "Disk Drive" published Apr. 18, 1984, by Data Technology Corporation for further details on 192 tpi disk drives employing the embedded servo system for accurate head positioning and track following.
I am aware that the 192 tpi disk drive has been compatible with the 96 tpi drive, as the former has been adapted for 96 tpi by writing on only the odd- or even-numbered ones of 192 tpi tracks. The 96 tpi disk thus formed by the 192 tpi drive can be read by the same 192 tpi device without any difficulties.
A problem does, however, arise when the 96 and 192 tpi disk drives are put to use in the following manner. Let us assume that the 96 tpi drive writes on a disk. Part or all of the data thus written at 96 tpi is then rewritten by the 192 tpi drive that has been adapted for the 96 tpi capability. Then the rewritten disk is read by the 96 tpi drive. During the rewriting of the 96 tpi data by the 192 tpi drive, as heretofore constructed, the old data has not been completely erased because of the difference in width between 96 tpi tracks and 192 tpi tracks. Typically, 96 tpi tracks are 155 micrometers in width whereas 192 tpi tracks are 119 micrometers in width, so that 36 micrometers of each old 96 tpi data tracks is left unerased. When this rewritten disk is read by the 96 tpi disk drive, its read/write head unavoidably scans not only the new data tracks but also the incompletely erased old tracks.